NASA Technical Reports Server (NTRS) 20150003109: Fabrication of Nanovoid-Imbedded Bismuth Telluride with Low Dimensional System

US008529825B2

d2) United States Patent

Chu et al.

(io) Patent No.: US 8,529,825 B2

(45) Date of Patent: Sep. 10, 2013

(54) FABRICATION OF NANOVOID-IMBEDDED
BISMUTH TELLURIDE WITH LOW
DIMENSIONAL SYSTEM

(75) Inventors: Sang-Hyon Chu, Newport News, VA
(US); Sang H. Choi, Poquoson, VA
(US); Jae-Woo Kim, Newport News, VA
(US); Yeonjoon Park, Yorktown, VA
(US); James R. Elliott, Vesuvius, VA
(US); Glen C. King, Yorktown, VA
(US); Diane M. Stoakley, Ashland, VA
(US)

(73) Assignees: National Institute of Aerospace

Associates, Hampton, VA (US); The
United States of America as
represented by the Administration of

NASA, Washington, DC (US)

( * ) Notice: Subject to any disclaimer, the term of this

patent is extended or adjusted under 35
U.S.C. 154(b) by 185 days.

(21) Appl.No.: 12/928,128

(22) Filed: Dec. 3, 2010

(65) Prior Publication Data

US 201 1/01 17690 Al May 19,2011

Related U.S. Application Data

(62) Division of application No. 11/831,233, filed on Jul.
31, 2007, now Pat. No. 8,020,805.

(60) Provisional application No. 60/834,547, filed on Jul.
31,2006.

(51) Int.Cl.

B28B 1/00 (2006.01)

(52) U.S. Cl.

USPC 264/620; 264/614

(58) Field of Classification Search

USPC 264/614, 620

See application file for complete search history.

(56) References Cited

U.S. PATENT DOCUMENTS

4,491,679 A * 1/1985 Moore 136/203

4,686,320 A * 8/1987 Novak et al 136/239

5,487,952 A * 1/1996 Yooetal 428/552

6,849,361 B2 * 2/2005 Fukudaetal 429/235

8,083,986 B2 * 12/2011 Choietal 264/620

2003/0032709 Al * 2/2003 Toshimaetal 524/439

2007/0240749 Al * 10/2007 Ohtaki 136/200

2009/0004086 Al * 1/2009 Kuhling et al 423/276

2009/0185942 Al* 7/2009 Choietal 419/30

FOREIGN PATENT DOCUMENTS
WO WO 2007/077065 * 7/2007

* cited by examiner

Primary Examiner — Joseph S Del Sole

Assistant Examiner — Russell Kemmerle, III

(74) Attorney, Agent, or Firm — Kimberly A. Chasteen

(57) ABSTRACT

A new fabrication method for nanovoids-imbedded bismuth
telluride (Bi — Te) material with low dimensional (quantum-
dots, quantum-wires, or quantum-wells) structure was con-
ceived during the development of advanced thermoelectric
(TE) materials. Bismuth telluride is currently the best -known
candidate material for solid-state TE cooling devices because
it possesses the highest TE figure of merit at room tempera-
ture. The innovative process described here allows nanom-
eter-scale voids to be incorporated in Bi — Te material. The
final nanovoid structure such as void size, size distribution,
void location, etc. can be also controlled under various pro-
cess conditions.

23 Claims, 22 Drawing Sheets

Figure of Merit (ZT)

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Sheet 1 of 22

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Year

FIG.1

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1

FABRICATION OF NANOVOID-IMBEDDED
BISMUTH TELLURIDE WITH LOW
DIMENSIONAL SYSTEM

CROSS-REFERENCE TO RELATED 5

APPLICATIONS

This application is a Divisional application of prior pend-
ing U.S. patent application Ser. No. 1 1/831,233 filed Jul. 31,
2007 now U.S. Pat. No. 8,020,805 and published Mar. 19, 10
2009 as U.S. 2009/0072078, which claims the benefit of U.S.
provisional application 60/834,547, with a filing date of Jul.

31, 2006. The entire disclosure of the prior application is
hereby incorporated by reference herein in its entirety. This 15
invention was made in part by employees of the United States
Government and may be manufactured and used by or for the
Government of the United States of America for governmen-
tal purposes without the payment of any royalties thereon or
therefor. 20

STATEMENT REGARDING FEDERALLY
SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government has a paid-up license in this inven- 25
tion and the right in limited circumstances to require the
patent owner to license others on reasonable terms, as pro-
vided for by the terms of Contract No. NCC-1-02043
awarded by the National Aeronautics and Space Administra-
tion, and Science and Technology Corporation Contract Nos. 30
L-71200D and L-71407D.

BACKGROUND OF THE INVENTION

1 . Field of the Invention 35

This invention generally relates to a fabrication method for

a thermoelectric material. More specifically, the invention
relates to a fabrication method for a nanovoid-imbedded bis-
muth telluride with a high figure of merit.

2. Description of the Related Art

To date, void-incorporated thermoelectric (TE) materials
have been studied in only a few compound systems, such as
bismuth, silicon, Si — Ge solid solutions, Al-doped SiC,
strontium oxide and strontium carbonate. Si — Ge samples 45
prepared by Pulverized and Intermixed Elements Sintering
(PIES) method exhibited 30% increase in TE performance
with 15-20% void fraction. Based on recent experimental
research, theoretical calculations also indicated that it is pos-
sible to increase the ZT of certain materials by a factor of 50
several times their bulk values by preparing them in 1 D or 2D
nanostructures. Bi — Te materials, especially with low-di-
mensional system, have been fabricated through solvo-ther-
mal method (1 D or 2D nanocrystals), metal-organic chemical
vapor deposition (MOCVD) (2D superlattice structure), elec- 55
trodeposition in porous alumina substrates (ID nanowire),
and reverse micelle method (0-D quantum dots).

Typical void sizes in most of prior-art studies were in the
micrometer range and no appreciable reduction in thermal
conductivity was realized. Lower thermal conductivity con- 60
tributes to the thermoelectric performance and in the prior-art
studies, no noticeable phonon disruption was observed. Aside
from theoretical predictions, there are no TE materials, based
on Bi — Te, that have demonstrated such an enhancement in
ZT values due to low-dimensional crystalline system. The 65
previous studies also showed most of voids in Bi — Te film
existed in an interconnected form which causes poor electron

2

mobility, resulting in lower electrical conductivity and hence,
lower thermoelectric performance.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to
provide a fabrication method for a thermoelectric material.

Another object of the present invention is to provide a
fabrication method for a thermoelectric material having a
high figure of merit.

Another object of the present invention is to provide a
fabrication method for a thermoelectric material having
increased electrical conductivity.

Another object of the present invention is to provide a
fabrication method for a thermoelectric material having
reduced thermal conductivity.

Another object of the present invention is to provide a
fabrication method for nanovoid-imbedded bismuth telluride
with low dimensional system.

Another object of the present invention is to provide a
fabrication method for nanovoid-imbedded bismuth telluride
with a high figure of merit.

Other objects and advantages of the present invention will
become more obvious hereinafter in the specification and
drawings.

In accordance with the present invention, a method for
forming high figure of merit thermoelectric materials is dis-
closed. The method includes providing nanocrystals of bis-
muth and tellurium and preparing a void generator material
including a plurality of nanoparticles each having a metallic
outer coating. The void generator material is preferably an
organic material and more preferably a ferritin protein. A
solution mixture of the bismuth nanocrystals, tellurium
nanocrystals and the void generator material is prepared and
deposited onto a substrate, preferably silicon. The deposition
method may be spin-coating, dipping, or solvent casting or
any other appropriate method. The deposited solution mix-
ture is heated in an oxygen environment to create a plurality
of nanovoid structures from the nanoparticles to provide a
nanovoid incorporated bismuth-tellurium film. In the pre-
ferred method, the deposited solution mixture is heated to no
more than 400° C. for approximately one hour in an oxygen
environment which is 99.999% pure oxygen. Following the
heating, the film is treated to remove any oxygen components
remaining from heating the mixture in the oxygen environ-
ment and to cause the formation of a crystalline structure in
the film. Such treatment is preferably accomplished by hydro-
gen calcination and hydrogen plasma quenching.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present inven-
tion, including other objects and advantages, reference
should be made to the Detailed Description of the Invention
set forth below. This Detailed Description should be read
together with the accompanying drawings, wherein:

FIG. 1 is a graph depicting the History of Thermoelectric
Figure of Merit (ZT);

FIG. 2 is an illustration of a morphological design of an
advanced thermoelectric material to enhance the ZT by
increasing electrical conductivity while concurrently reduc-
ing thermal conductivity by populating nanovoids into the
material, which material can be utilized in at least one
embodiment of the present invention;

FIG. 3 is a graph of the figure of merit of several TE
materials which could be utilized in at least one embodiment
of the present invention;

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3

FIG. 4 is a flow chart of the fabrication of a nanovoid
bismuth telluride (Bi — Te) thermoelectric material that could
be utilized in at least one embodiment of the present inven-
tion;

FIG. 5 is a diagram depicting a portion of the fabrication 5
process of the Bi — Te thermoelectric material referenced in
FIG. 4, i.e., the formation of low-dimensional bismuth tellu-
ride nanocrystals using solvo-thennal process;

FIG. 6 is a diagram depicting a portion of the fabrication
process of the Bi — Te thermoelectric material referenced in 10
FIG. 4, i.e., a three-phase mixture of Be — Te NCs, voigens,
and cosolvent;

FIG. 7 is diagram depicting a portion of the fabrication
process of the Bi — Te thermoelectric material referenced in 15
FIG. 4, i.e., fabrication of nanovoid Bi — Te material by depo-
sition and pyrolysis process that creates nanovoid structure in
thermoelectric material;

FIG. 8A is an image of two-dimension nanosheets fabri-
cated using bismuth telluride crystals which can be utilized in 20
at least one embodiment of the present invention;

FIG. 8B is a graph of element analysis data by EDAX of
same the nanosheets shown in FIG. 8A;

FIG. 9 is an image of a bismuth telluride disk fabricated by
a cold press method, which can be utilized in at least one 25
embodiment of the present invention;

FIG. 10 is a perspective view of one possible embodiment
of the Advanced Thermoelectric (ATE) energy conversion
system of the present invention;

FIG. 11 is a flow chart depicting the cascaded efficiency of ,( 1
a 3-layer ATE system in a tandem mode in accordance with at
least one embodiment of the present invention;

FIG. 12 is a graph depicting the efficiencies of state-of-the
art solar, and thermoelectric cells; 35

FIG. 13A is a perspective view of a flattened airship with
ellipsoidal cross-section to maximize the reception of solar
flux in accordance with at least one embodiment of the
present invention;

FIG. 13B depicts the ellipsoidal cross section of the airship 40
shown in FIG. 13 A;

FIG. 13C is an expanded view of the solar troughs disposed
on the top surface of the airship shown in FIG. 13A, utilizing
the ATE conversion system;

FIG. 14 is a side-view drawing of another embodiment of 45
an HAA of the present invention;

FIG. 15 is the cross-sectional view of the HAA along the
plane indicated by dashed line A- A in FIG. 14;

FIG. 16 are front views of an HAA, depicting a method of
operation of the HAA, in accordance with at least one 50
embodiment of the present invention;

FIG. 17 is a drawing of the troughs shown in FIG. 13C,
depicting ATE power modules utilized with linear parabolic
troughs to collect solar flux, in accordance with at least one
embodiment of the present invention; 55

FIG. 18A is a drawing of a flexible thin-film rectenna array
that, in at least one embodiment of the present invention, can
be attached under the bottom surface of an HAA to receive
and convert microwave power into DC Power;

FIG. 18B illustrates an example of a circuit that could be 60
utilized in the rectemias depicted in FIG. 18A;

FIG. 19 is a logic diagram of potential microwave power
use by an HAA equipped with a rectenna array, such as the
array depicted in FIG. 18;

FIG. 20 is flow chart depicting a possible power distribu- 65
tion scenario of an HAA for various application devices that
might be onboard;

4

FIG. 21 is a perspective view of a novel unmanned or
maimed aerial (UAV or MAV) configured for electric propul-
sion power to be wirelessly transmitted; and

FIG. 22 is an image depicting an HAA utilizing a sophis-
ticated relay system of laser power.

DETAILED DESCRIPTION OF THE INVENTION

The present invention comprises incorporation of control-
lable nanovoid structures into bismuth telluride (Bi — Te)
thermoelectric materials using Bi — Te nanocrystal process,
in order to achieve high figure of merit for TE devices. One
advantageous embodiment of the present inventive system, or
device, is based on advanced thermoelectric (ATE) materials
which can be developed for a targeted figure of merit (FoM)
goal, advantageously, greater than 5. This inventive power
technology enables many application specific scenarios
which might not have been possible with prior technology.

As shown in FIG. 1, the FoMs of most TE materials devel-
oped to date are still below 2. A FoM of 1 is equivalent to a
thermal to electrical energy conversion efficiency greater than
6%. To achieve advanced TE (ATE) materials 20 with a high
FoM, nanovoids 21 can be incorporated, as shown in FIG. 2,
into the TE material s 23 to increase the electrical conductivity
(EC) while reducing the thermal conductivity (TC). The
nanovoids 21 are essentially nano spherical shells 24 having
internal voids 22. In one embodiment, spherical shells 24 of
approximately 1 0 nm to approximately 20 nm in outer diam-
eter can be made from a metallic component such as gold or
cobalt using a bio-template, as discussed in more detail
below. The void 22 inner diameter can range from approxi-
mately 10 mn to approximately 12 nm. The void diameter
needs to be small enough to avoid a reduction in electrical
conductivity and a change in morphology of the bulk material
to poly-crystalline.

Typically, within a crystalline structure of a material, the
heat transfer mechanism is mainly dictated by phonon trans-
mission (>70%) ratherthan by energetic electrons (<30%) for
temperatures below 900 K. Accordingly, amethodto manipu-
late the phonon transmission within a crystalline medium
offers a capability to control the thermal conductivity. The
metallic nanovoids 21 populated inside bulk matrix TE mate-
rials 23 create large phonon scattering cross-sections that
effectively block the transfer of thermal energy through them.
The TE material 23 in an unoccupied area 25 that is sand-
wiched between nanovoids 21 will become a phonon 26
bottleneck since the narrowly sandwiched bulk material is
under a high tension induced from the spherical formation of
the TE material boundary around the nanovoids, as shown in
FIG. 2. The phonon bottleneck is essentially the combination
of unoccupied area 25 and the nano voids 21. The individual
phonon 26 merely indicates that the phonon 26 cannot propa-
gate easily through the unoccupied area 25. The structure of
the TE material boundary surface around the spherical nano-
voids 21 may be framed of a high energy bonding group that
develops high tension over the surface. The material structure
with high tension would be less subjected to an oscillatory
mode transmission than material structures in normal tension.
Therefore, both the high tension and the narrow passage 25
between nanovoids are resistive to phonon transmission and
accordingly regarded as a phonon bottleneck. Ultimately, the
imbedded nanovoids 21 create the phonon scattering cross-
sections and bottlenecks throughout the matrix material as
shown in FIG. 2.

The selection basis of TE materials for the present inven-
tion can be according to the temperature at which the perfor-
mance of the particular TE material is best-suited. For

US 8,529,825 B2

5

example, in at least one advantageous embodiment of the
present invention, the materials selected can be silicon-ger-
manium (SiGe) and bismuth telluride (Bi 2 Te 3 ), along with
lead telluride (PbTe). Currently pending U.S. patent applica-
tion Ser. No. 1 1/242,41 5, entitled “Silicon Germanium Semi- 5
conductive Alloy and Method of Fabricating Same, by Park,
et al., filed on Sep. 27, 2005, and hereby incorporated by
reference as if set forth in its entirety herein, discloses addi-
tional detail relative to a lattice-matched silicon germanium
semiconductive alloy and its fabrication that is suitable for to
use in at least one layer of the present invention. Materials
desirable for use would also include nanovoid-imbedded
forms of Si Ge, PbTe, and Bi 2 Te 3 .

Nanovoid-imbedded Bi — Te would be suitable for use in
one or more different layers. Bismuth Telleride is currently 15
used in solid-state TE cooling devices due to its high figure of
merit at room temperature, as shown in FIG. 3, but its appli-
cations are still limited by poor TE properties. To improve TE
performance, nanometer-scale voids can be incorporated into
Bi — Te material, with the void size, size distribution, void 20
location, etc. controlled under various process conditions.
The nanovoids reduce thermal conductivity by disrupting
phonons without sacrificing electron transport, thereby
allowing for the reduction of thermal conductivity while
increasing electrical conductivity. The nanovoid incorpora- 25
tion is controlled by thermodynamic miscibility and kinetic
mobility of two phases, TE precursor and voigen. Metal nano-
particles such as but not limited to gold, cobalt, platinum,
manganese, and iron, anchored on the voigen material surface
eventually form a metal layer or lining through an annealing 30
process. The spherical void by metal lining becomes a pas-
sage of mobile electrons and aids the electrons to move
through the nanovoid structure.

There are several methods for imbedding nanovoids into a
matrix material. For example, in the solvo-thennal method 35
which can be used for Bi 2 Te 3 , the nanocrystals of bismuth
telluride are created and then mixed with nanovoids before
solvent casting. Through solvent casting on a substrate after
mixing, a cake of bismuth telluride is made. This cake goes
through calcination and hydrogen plasma etching processes 40
to remove unwanted impurity elements. Finally, the sponge
form of bismuth telluride is sintered to become a matrix of
single crystal deposition.

To date, the void-incorporated TE materials have been
studied in several compound systems, such as bismuth, sili- 45
con, Si — Ge solid solutions, Al-doped SiC, strontium oxide
and strontium carbonate. Si — Ge samples prepared by Pul-
verized and Intermixed Elements Sintering (PIES) method
exhibited 30% increase in TE performance with 1 5-20% void
fraction. Theoretical calculations also indicate that it is pos- 50
sible to increase the ZT of certain materials by a factor of
several times their bulk values by preparing them in 1 D or 2D
nanostructures. Bi — Te materials, especially with low-di-
mensional system, have been fabricated through solvo-ther-
mal method (1 D or 2D nanocrystals), metal-organic chemical 55
vapor deposition (MOCVD) (2D superlattice structure), elec-
trodeposition in porous alumina substrates (ID nanowire),
and reverse micelle method (0-D quantum dots).

Nanovoids are incorporated, in a controllable maimer, into
bismuth telluride (Bi — Te) thermoelectric materials using a 60
Bi — Te nanocrystal process, in order to achieve a high figure
of merit. The fabrication process is shown in FIG. 4 . An
important element in developing high figure of merit TE
materials is to fabricate void generators (“voigens”) and to
populate voigens into the bulk TE materials. The population 65
distribution of voigens into bulk TE materials detennines the
reduction level of thermal conductivity by setting up phonon

6

bottlenecks between voigens where phonon scattering takes
place. As such, the population density will depend on the
desired thermal conductivity.

First, precursor materials are prepared for syntheses of
Bi — Te nanocrystals 40 . Various Bi — Te nanocrystals can be
prepared by employing the solvo-thennal process. Nanocrys-
tals of various sizes and shapes can be made by changing
synthesis conditions, as illustrated in FIG. 5 . Possible geom-
etries of nanocrystals include nanorod, nanosheet, nano-
sheet-rod, nanorag, etc.

Another important part of material preparation is to syn-
thesize voigen materials 41 . Voigen materials are designed to
meet several important roles for high figure of merit TE
materials: (1 ) phonon scattering centers, (2) reduced thermal
conductivity, and (3) enhanced electrical conductivity. Cre-
ation of nanovoids with size and shape uniformity is also an
important issue that will determine void fraction within bulk
TE materials. To enhance electrical conductivity, voigen
materials (e.g., ferritin protein), which are in a nano-scale,
generally approximately 8 nm inner diameter and 1 2 nm outer
diameter are coated with a metal lining, such as but not
limited to gold, cobalt, platinum, manganese, and iron. While
the voigen material is not limited to ferritin protein and may
be other bio-templates, the ferritin protein is generally desir-
able due to its ability to form the spherical shape with void.
The metal nanoparticles anchored on the voigen material
surface eventually form a metal layer or lining through an
annealing process. The annealing process comprises hydro-
gen calcination and hydrogen plasma quenching. The metal
lining is approximately several atomic layer thick and gener-
ally no more than approximately 3 to 4 nanometers. Nanom-
eter-scaled voigen molecules can be dispersed in a cosolvent
system with Bi — Te nanocrystals 42 , as illustrated in FIG. 6.
The nanoscale phase separation between the precursor and
voigen is induced by their thermodynamic miscibility, and
determines the final nanovoid structure. The diameter of voi-
gen materials is designed to be less than approximately 20 nm
after coated with the metal nanoparticles. The metal nanopar-
ticles will remain as a metal lining that forms a spherical void
after a pyrolysis process. The metal lining that forms the
spherical void forms the passage for electrons which provides
for high electrical conductivity. In conjunction with the
reduced thermal conductivity attributed to the phonon scat-
tering of nanovoids, the enhancement of electrical conductiv-
ity is desirable for high figure of merit TE performance.
Overall TE performance can be possibly deteriorated by the
aggregation of metallic spherical voids without spreading
within the bulk material. Accordingly, the distribution of
nanovoids throughout the TE material is important.

The mixture of the three-phase system (bismuth, tellurium,
and voigens) is deposited on a substrate using solution-based
thin-film coating methods. Known substrates, suchas silicon,
are suitable. Suitable coating methods include spin-coating,
dipping, solvent casting, etc. 43 . After coating or casting, the
films are placed in a vacuum chamber and heated under the
environment of ultra pure hydrogen (approximately
99.999%) 44 . A temperature of less than approximately 400°
C. is generally desirable. The heating time is dependent on the
thickness of the film, but generally approximately one hour.
Voigen material which is organic, such as ferritin protein, and
other organic components from the solution used are ther-
mally decomposed and removed during pyrolysis process 44
until nanovoid-incorporated Bi — Te film 46 is obtained, as
illustrated in FIG. 7. Hydrogen calcination and hydrogen
plasma quenching 45 are then used, which first remove oxy-
gen components that are the residue of organic breakdown

US 8,529,825 B2

7

with hydrogen plasma and second make crystalline structure
in Bi — Te films through the annealing process, respectively.

A solvothermal pre-process to make the Bi 2 Te 3 nanopar-
ticles successfully produced black powder of bismuth tellu-
ride crystal with low dimension. FIG. 8A shows two-dimen-
sional nanosheets 80 fabricated using potassium hydroxide
(KOH) and ethylenediamine tetraacetic acid (EDTA), follow-
ing the synthesis process described in FIG. 5. The nanosheets
varied in size, all thicker than approximately 30 mn, and
Energy dispersive X-ray spectroscopy (EDAX) analysis
using FE-SEM (Field Emission Scanning Electron Micros-
copy) confirmed the existence of Bi 2 Te 3 by element energy
analysis, as shown in FIG. 8B. The thickness of the nanosheet
is dependent on the repetition of the coating process.

Disk-type samples were prepared using a cold press
method at room temperature. FIG. 9 shows the bismuth tel-
luride disk 90 fabricated with its nanocrystal powder. The
nanoscale, nanovoid structure causes phonon scattering with-
out disturbing electron mobility, thus increasing the figure of
merit from low-dimensional nanocrystal Bi — Te materials.

The same or similar skills and/or techniques used to fabri-
cate the nanovoid-imbedded Bi 2 Te 3 ATE material described
above can be readily extended to the fabrication of other
nanovoid-embedded TE materials, such as cobalt antimonide
(CoSb 3 ) and leadtelluride (PbTe), for optimal thermoelectric
performance at temperature ranges different from Bi 2 Te 3 and
SiGe. FIG. 3 shows several TE materials along with their
associated best-suitable temperature ranges.

The present inventive energy conversion system is not lim-
ited to the above-noted layer materials. Other TE materials,
and nanovoid embedded TE materials would also be suitable,
and would be chosen based on the specific applications and
temperature ranges. In general, suitable materials will have a
high Seebeck coefficient, a high electrical conductivity and a
low thermal conductivity.

In accordance with the present invention, these ATE mate-
rials can be utilized in the manufacturing of ATE energy
conversion devices and systems, as illustrated in FIG. 10. To
make a p-n junction for TE energy conversion, the TE matrix
materials developed can undergo conventional doping pro-
cesses to create N-type and P-type materials (i.e., semicon-
ductors).

As understood in the art, “doping” refers to the process of
intentionally introducing impurities into an intrinsic semi-
conductor in order to change its electrical properties. A P-type
semiconductor is obtained by carrying out a process of dop-
ing wherein a certain type of atoms are added to the semicon-
ductor in order to increase the number of free positive charge
carriers. When the doping agent (acceptor material) is added,
it accepts weakly-bound outer electrons from the semicon-
ductor’s atoms, and creates holes (i.e., atoms that have lost an
electron). The purpose of P-type doping is to create an abun-
dance of such holes. When these holes move away from its
associated negative-charged dopant ion, one proton in the
atom at the hole’s original location is now “exposed” and no
longer cancelled by an electron, resulting in a hole behaving
as a quantity of positive charge. When a sufficiently large
number of acceptor atoms are added, the holes greatly out-
number the thermally-excited electrons. Thus, the holes are
the majority carriers in P-type materials, and the electrons are
the minority carriers. In contrast, an N-type material is
obtained by adding a doping agent known as a donor material,
which donates weakly bound outer electrons to the semicon-
ductor atoms. For example, an impurity of a valence-five
element can be added to a valence-four semiconductor in
order to increase the number of free mobile or carrier elec-
trons in the material. These unbound electrons are only

8

weakly bound to the atoms and can be easily excited into the
conduction band, without the formation of a “hole,” thus the
number of electrons is an N-type material far exceeds the
number of holes, and therefore the negatively charged elec-
5 trons are the majority carriers and the holes are the minority
carriers.

Non-limiting examples of doping material that could be
used in the instant invention are boron and phosphor, which
doping could be done in a known maimer, such as by ion
to implantation or diffusion. In general, an array of pairs of p-n
junction materials, or elements, is utilized to increase the
thermal exposure area. As shown in FIG. 10, a TE module 100
can consist of three layers of p-n-junction arrays 102, 103,
104 in a tandem mode that operate most efficiently at high,
15 medium, and low temperatures, correspondingly in a tandem
mode, providing a cascaded conversion efficiency. In at least
one advantageous embodiment, these TE layers 102, 103, 104
comprise advanced TE materials, making a cascaded effi-
ciency greater than about 60 percent obtainable, as indicated
20 in FIG. 11. While the shown embodiment consists of three
layers, it should be understood that depending upon the
desired application, the number of layers can be varied
accordingly. Additionally, these layers can be assembled in
the maimer known in the art, for example, a metallized
25 ceramic 105 could potentially be layered between them.

Such a tandem arrangement allows efficient energy liar-
vesting from a heat source, thus allowing the present inven-
tive energy conversion system to be effectively utilized in
High Altitude Airship (HAA) applications where solar energy
30 is considered as an energy source. In order to address the
power-related requirements for lighter-than-air vehicles,
including airships and hybrid fixed-wing configurations, the
integration of the ATE devices can take on many different
forms, dependent on configuration needs and mission
35 requirements. As explained above, in general, an array of a
pair of p-n junction materials is necessary to increase the
thermal exposure area, and the ATE energy conversion device
100 may consist of multiple layers of p-n-junction arrays, as
shown in FIG. 10. The first, ortop, layer 102 is built from the
40 array of thermoelectric material segments 101 that operate
optimally at the higher temperatures, such as SiGe. The sub-
sequent layers, or stages, are driven by the propagating ther-
mal energy of the preceding layer. The selection of the appro-
priate thermoelectric material for each of these subsequent
45 layers is chosen dependent on the optimal thermoelectric
figure of merit, ZT, for the respective operating temperature
ranges. Referring again to the general example as shown in
FIG. 10, the second and third layers 103, 104 of this 3-layer
thermoelectric power system 100 are respectively built from
50 PbTe and Bi 2 Te 3 in a regenerative cycle mode of operation.
Further, in at least one embodiment of the invention, it is
desirable to utilize ATE materials such as nanovoid embedded
Bi 2 Te 3 and PbTe, and lattice-matched SeGi as the three lay-
ers. The invention, however, is not limited to these materials
55 and may use only one or more layers of ATE materials.

In operation, the incident solar flux first heats up the initial
layer 102 which is built with an optimized high temperature
thermoelectric material. The unused thermal energy from the
first layer is subsequently utilized by the second layer 103
60 which is built with an optimized mid-temperature thermo-
electric material, such as PbTe. Repeating this process again,
the third layer 104, such as Bi 2 Te 3 , uses the unused energy
from the second layer to maximize the conversion of the
energy that is otherwise underutilized. With this repeated
65 process, the number of different thermoelectric material lay-
ers needed, also depends on the overall temperature range
available and desired. With the available thermal energy from

US 8,529,825 B2

9

the solar flux, the ATE can harness more energy than photo-
voltaic cells that use quantized electrons of the photons from
the solar flux. Hie integration of advanced TE materials can
provide significant levels of electrical energy because of this
cascaded efficiency of multiple-layer TE modules 100 that 5
are much higher than the efficiency of a single layer and the
broad use of the solar thermal energy. The layered structure of
the advanced TE materials is specifically engineered to pro-
vide maximum efficiency for the corresponding range of
operational temperatures. A representative three layer system to
of advanced TE materials, as shown in FIG. 10, generally
operates at high, medium, and low temperatures, correspond-
ingly in a tandem mode. The cascaded efficiency of such an
arrangement is estimated to be greater than 60% as indicated
in FIG. 11. With multiple advanced thermoelectric material 15
stages, a highly effective and efficient energy harvesting sys-
tem may then be optimized for representative operational
requirements such as maximum power, minimum weight,
minimum size, etc.

As mentioned above, these ATE materials can be chosen to 20
fit specific applications depending upon the overall tempera-
ture range available and desired. This same procedure can be
used to construct specialized TE energy conversion devices in
accordance with the present invention, for many different
applications, including a variety of different heat sources. 25
That is to say, by understanding each application prior to
fabricating the inventive TE energy conversion device, the
appropriate ATE materials can be chosen and layered (e.g.
depending upon the original thermal load and a calculation of
how much heat must be removed from this load by each layer, 30
to achieve the desired performance objectives for each layer
and overall).

Additionally, as would be known to one with ordinary skill
in the art, the thickness of each layer can also be varied (for
example, by increasing the number of sub-layers of the ATE 35
material) until a desired temperature reduction is achieved
prior to the thermal energy passing into the next layer, so that,
optimally, when the energy is passed to the next layer it is at
a temperature that will permit peak, or close to peak, thermal
energy conversion performance by the receiving layer. Typi- 40
cally, a layer thickness might range from less than 1 mm to
several millimeters, or more, in thickness, depending upon
the material and application.

In this inventive fashion the overall efficiency of the system
increases beyond that achieved by known methods (for 45
example, where only one known TE material is used). Addi-
tionally, the ATE devices of the present invention become
more effective than solar cells because the performance of
solar cells is monolithically tied to band-gap energy structure,
so that they only couple with certain spectral lines. Also, the 50
higher the efficiency of the solar cells, the higher the cost and
complexity of fabrication. For comparison purposes, FIG. 12
shows the layout of predicted figure of merits as a goal to
achieve, which is added onto an existing diagram of solar cell
efficiencies. As compared to solar cell technology in efifi- 55
ciency, the ATE system is competitive. However, considering
the available energy from solar flux, the ATE system, using
thermal energy, can harness more energy than photovoltaic
cells that use the quantized electrons by photons from solar
flux. 60

While solar energy conversion is discussed in detail herein,
and specifically in reference to HAAs, it should be reiterated
that the instant power conversion invention is not so limited in
scope, rather the instant ATE energy conversion system can
be used to harvest heat from a wide variety of sources (e.g., 65
power plants, radioisotopes, automotive cooling systems,
etc.) for many different energy generation and/or cooling

10

applications. Additionally, the completion of the fabrication
of the final circuitry and fabrication of an operable thermo-
electric conversion system using the inventive system dis-
closed herein, would be understood by someone with ordi-
nary skill in the art as these techniques are well-known in the
art.

There are several potential candidate energy harvesting
technologies for HAAs, such as solar cells, fuel cells, Sterling
engines, and TE generators. Due to the above-mentioned
restrictions, ATE devices are extremely attractive because of
the cascaded efficiency of the multi-layer TE modules of the
present invention, that are much higher efficiency than the
efficiency of a single layer and the broad use of solar thermal
energy. As explained above, the layered structure of the ATE
materials is specifically engineered to provide maximum effi-
ciency for the corresponding range of operational tempera-
nires. The present invention essentially functions like regen-
erative cycles in tandem. Such a highly effective energy
harvesting feature of this tandem system based on multiple
layers of advanced TE materials can be the basis of an HAA
power budget plan.

In accordance with the present invention, to’maximize the
reception of solar thermal energy, an ellipsoid cross-sec-
tioned high altitude airship (HAA) 135 has been designed, as
shown in FIG. 13 A. In at least one embodiment, such an HAA
can be 1 50 meters long, 60 meters wide, and 24 meters high.
In size, this HAA is about 2.5 times larger than a Goodyear®
blimp, which is 60 meters long. With this dimension of HAA
135, the perpendicularly incident solar power amounts to
about 9 MW. However, the daytime exposure varies with sun
location. As shown in FIG. 16 (a cross-sectional view), if it is
necessary, the HAA 135 can be reoriented to receive the
maximum solar energy by keeping the top surface 135a of the
HAA 135 always substantially perpendicular to the solar
angle 160. In at least one embodiment, a vectored electric
propulsion system can be used for this purpose.

When the top surface 135a of HAA 135 follows the sun, the
power management and control (PMC) station 141 installed
under the belly of HAA 135 is designed to move on a guide
rail 140 (shown in FIG. 14), to reposition itself, always dan-
gling at, or near, the bottom, or lowest point, of HAA 135 for
every collector orientation. FIG. 16 shows the repositioned
PMC 141 at the nadir point of HAA 135 along with the sun
position 160. In such a manner, the energy harvested from
sun-rise to sun-set becomes effectively maximized, regard-
less of exposure variation over the course of the day. Using
20% efficient photovoltaic (PV) cells, the maximum con-
verted power would be less than 2 MW. With the inventive
advanced TE system of the same efficiency, the converted
power would be greater than 4 MW because the cascaded
efficiency of three layers is calculated to be approximately
49%. Considering a three-layered structure of the advanced
TE materials having a FOM 5, the cascaded efficiency
amounts to be close to 66% (see FIG. 11). If the amount of
losses (35%) due to geometrical orientation (23%), reflection
(7%), absorption (3%), and transmittance (2%) is considered
for the estimation of cascaded efficiency, the total harvestable
unit becomes 0.427 under the condition of 0.6512 input
instead of 112 used in FIG. 11. Accordingly, the obtainable
power amounts to be 3.84 MW which is substantial to accom-
modate several roles of the HAA. FIG. 13A depicts some of
the scenarios that might be feasible, such as feeding power to
off-shore or isolated locations, for example, in lieu of having
to build expensive power stations. The HAA can also become
a mothership to wirelessly feed power to deployed unmanned
vehicles.

US 8,529,825 B2

11

Referring to FIGS. 13, 15 and 17, in one embodiment of the
present invention, the ATE power module 100 (FIG. 10) can
be used in conjunction with linear parabolic troughs 130.
These troughs 130 can have a 300 cm aperture width to collect
solar power, as shown in FIG. 17. In at least one application,
the back-surface 170 ofATE strips 100a is reflective to reduce
solar energy absorption and faces outside directly to the cold
environment of high altitude to drop the surface temperature
by convective cooling. The temperature at 70,000 feet or
above in the atmosphere is extremely cold and hovers below
-73° C. Accordingly, to maximize the performance of the
ATE system, the solar trough concentrators are used to focus
solar flux to the surface 171 of the 1 st layer that faces the
reflector trough 130 while the back side 170 of the V d layer
faces the cold atmosphere to increase the temperature gradi-
ent. In at least one embodiment, the material of the reflector
trough 130 can be, for example, enhanced aluminum coated
thin-film membrane which is sufficiently hardened to main-
tain its parabolic shape. Each reflector 130 can be covered by
a transparent membrane 151 that allows sun light to impinge
into the parabolic trough 130. The strip of ATE power module
100a is located on a focal line of the parabolic trough 130 and
connected to the transparent thin film window material 151,
for example, both edges of the strip 100a can be connected to
the transparent material 151. An additional advantage of this
type of ATE energy conversion system is that the structural
formation of such solar trough 130 will enhance the strength
of large sized HAA.

The nighttime power requirements of HAAs may not be
alleviated because the HAA’s nighttime operation frequently
has the same importance as their daytime operation. There-
fore, the power for nighttime operation typically must be the
same level as that of the daytime usage. Based on the daytime
figure for required power, three components of power infra-
structure are actively involved to supply necessary power.
That is to say, for nighttime, the power required can be aug-
mented from the onboard fuel cells, battery and a rectemia
array 180a that is attached at the bottom surface 1356 of HAA
135 (see FIGS. 14 and 15, where the rectenna array is indi-
cated by a dashed line). These combined systems provide at
least a megawatt level of power for the intermittent operation.

Hydrogen fuel-cells with the capacity of several hundreds
kilo-watt level are onboard for the nighttime power genera-
tion. The water which is an end product of fuel -cell process is
collected and dissociated into hydrogen and oxygen through
electrolysis process using the power harvested during day-
time. The hydrogen and oxygen is collected and fed back to
fuel cells later at nighttime.

The power stored in the thin-film battery during daytime
can be drained out for nighttime use. The battery storage
capacity (-600 Coulomb/gram) is proportional to its own
weight increase. Therefore, the battery is not regarded as the
major power provider for night time use. It can be used for
emergency purposes.

The arrays of thin-film rectennas 180, as shown in FIG.
18A, can be readily fabricated on a flexible film 181 which
can be used within the structural envelope of the HAA. In at
least one embodiment, the arrays 180/180a are patched under
the bottom surface 1356 of HAA 135 (see FIG. 15, which
depicts a cross-sectional view of the HAA shown in FIG. 14),
to receive and convert microwave power 152 into DC Power,
as illustrated schematically in FIG. 18B. The conversion effi-
ciency of rectennas is unusually high (-85%), but the collec-
tion efficiency is poor because of the dispersive nature of
microwave. The bottom surface area of HAA 135 is wide and
nearly flat to enable the HAA 135 to collect most of dispersed
microwave energy. At the 21 km (-70,000 ft) altitude, the area

12

required to collect the W-band (90-100 GHz) microwave is
approximately 48 meters in diameter. This number is calcu-
lated by the Gaubau relationship, which is defined by the
following formula:

5

\[a~a ~

T= I z

10

wherein: A,, is the area of receiving antenna; A, is the area of
transmitting antenna; Z is the distance between the transmit-
ting and receiving antennas; A. is the wavelength of micro-
wave; and x is the parameter determined for 100% reception
15 which, in the case for microwaves, is 3.

In at least one embodiment of the present invention, the
bottom surface 1356 of HAA 135 can be 1 50 meters long and
60 meters wide. Accordingly, the microwave power at the
W-band can be delivered to the array of rectennas 180a at the
20 bottom surface 1356 of HAA 135 almost all without loss.
FIG. 19 shows a logic diagram of microwave power use. The
power 191 received by rectemia arrays 180a is allocated and
distributed by the power allocation and distribution (PAD)
logic circuit 192 to specific nodal points where the power is
25 mostly needed, such as propulsion unit or subsystems 193.
Otherwise, the excessive power can be stored in an array of
thin-film solid-state batteries 194 for later use. Thus, a large
amount of microwave power can be delivered to the HAA 135
from a ground or a ship-board microwave power beaming
30 station. Even for a remotely dispatched HAA 135, wireless
airborne electro-refueling by airplanes is possible. Multiple
microwave stations combined can aim their beams onto a
rectenna-equipped HAA 135 to feed the power required for
the operation at night.

3 5 The power harvested by the inventive ATE generator can be

utilized for the power transmission to UAVs, for onboard
systems operation, and for internal power requirements such
as propulsion and control. FIG. 13A illustrates a graphical
scenario of operational mode of an HAA 135. If more power
40 is required, of course, in at least one embodiment, it is solved
by the enlargement of HAA 135, and the utilized ATE system
100a with troughs 130. The total power harvested (3.84 MW)
can be distributed for propulsion for stationary positioning
and maneuvering, power storage, microwave beaming for
45 MAV or UAV operations, laser power beaming to ground
locations, such as for illumination or telecommunication pur-
poses, and house-keeping activities. Such applications
require a continuous power source that will run for several
hours in a sequential or a pulse mode anytime throughout the
50 day and the night. FIG. 20 shows the power flow diagram
based on the power estimation that is to be harvested by the
ATE array placed on top of the HAA. The power allocated for
the operations of those onboard devices is estimated to give a
glimpse at one possible power picture.

55 The power harvested by the inventive ATE generator can
also be utilized for propulsion for position correction and
maneuver. The wind at an altitude of 21 km (70,000 feet) or
above is substantially lower than typical seasonal jet-streams
that exist within the northern hemisphere. Nevertheless, the
60 large cross-section of the HAA is vulnerable to drifting along
with wind. Continuous positioning and maneuvering opera-
tion of the HAA against the wind is necessary and crucial for
the stationary operation and maximum solar exposure over
solar angle variation. Otherwise, the HAA will drift away to
65 an undesirable location where the use of onboard devices may
be impossible. The propulsion for position correction and
maneuvering is also required during the night time.

US 8,529,825 B2

13

Another potential embodiment of the inventive HAA con-
figuration includes a base for UAV or MAV airships. A novel
lightweight, high performance, long endurance UAV con-
figuration, as shown in FIG. 21, has been developed that
combines a polymer structure with an electrical power gen- 5
erating system to produce new missions and capabilities for
air vehicles. As presently envisioned, this class of UAV sat-
isfies aeronautical missions for high altitude, characterized
by long endurance, electric propulsion, propellantless, and
emissionless. The configuration utilizes a polyimide struc- to
tural material for creating the primary wing and fuselage
elements of the vehicle. The polymer structure functions to
carry normal, bending, and pressure loads as experienced
from sea-level to cruising altitudes. The polymer structure
incorporates arrays of rectennas 1806 to form a wireless 15
power generation system. The rectenna system 1806 has been
demonstrated at microwave wavelengths (X-Band) to provide
275 volts from 18 milli-Watts of incident energy. The rect-
enna system 1806 can be designed for other and higher fre-
quencies depending on configuration requirements, atmo- 20
spheric transmissibility, etc. The resulting electrical energy
can be used as power for electrical motors for propulsion of
the UAV’s alone, in combination with electrical storage sys-
tems, or in combination with other hydrocarbon engines,
including hybrid modes of operation. 25

As conceived, the UAV is air-launched from and returned
to the HAA base. The HAA base for UAVs is built under the
HAA, as shown in FIGS. 14 and 15. The UAVs may also be
launched or retrieved by hand, machine, towing, or dropped
from other aircraft and/or helicopters. By nature of the struc- 30
tural material and concept of utilization, the UAV does not
require landing gear or skids. As such, the structural design
requirements for takeoff, landing, and taxiing are reduced or
eliminated and thereby relax the overall structural design
loads and requirements. 35

To sustain a long duration operation, the helium or helium/
hydrogen mixture filled fat-body airframe of UAVs is consid-
ered to reduce the power requirement for propulsion by both
reducing the body weight and increasing the lift force by
buoyancy. The fat-body framed UAV mode 210 to be pro- 40
pelled by electric motors is shown in FIG. 21. Two electric
motor driven propellers 211, 212 are located at both the
wing-tips and control the flight direction by changing rota-
tional speed. The power for these planes is obtained from
microwave through rectenna arrays 1806 that are integrated 45
on the skin of the airframe. The range of maneuver is deter-
mined by the envelope of microwave beaming column and the
guided direction of beam. As long as any MAVs or UAVs are
within the beam column, the power is continuously fed into
them. 50

Suppose that a UAV has a 10 m 2 rectenna arrays that are
integrated into the skin of fuselage and both wings as shown
FIG. 21. If 1 MW of microwave power as described in the
block diagram of FIG. 20 is transmitted at w-band, the power
flux density of microwave at the ground level will be approxi- 55
mately 60mW/cnf. The power received by a UAV which is a
20 km away to the ground level and has a 10 nf rectenna
arrays will amount to be 6 kW within the power beam column
of 50 meters in diameter. Using the same logic shown in FIG.

19, the power is allocated to the propulsion system and other 60
functional systems, such as probes. Suppose that the maneu-
ver of the UAV requires 4 kW of the received power. The rest
can be used for sensors and probes for other operations.
However, the power receiving area of MAVs or UAVs is
limited due to their own limited sizes. Therefore, they require 65
an extra lifting force to stay aloft. The helium-filled MAV or
UAV 210 as shown in FIG. 21 will gain an extra lifting force.

14

The UAV size of 5 m 3 helium filled will gain the buoyancy
force of 5 1 N which will reduce the weight by approximately
5 kg.

Another scenario for HAA use is for laser power transmis-
sion technologies for space applications which were devel-
oped in late 1970 through the 1980’s using the directly solar
pumped iodine laser and also a high power diode laser array.
The efficiencies of continuous wave (CW) lasers are, in gen-
eral, poor, especially for the short wavelength lasers. If we
consider a laser with 1 0% efficiency, the actual laser power to
be conveyed through the beam becomes 1 00 kW level. With a
pulse forming network, the laser power output would be much
higher to a few tera-watts (TW) level by pulse compression.
The reflectors 220 that are installed on a HAA can also be
used to relay the laser beam power 221 to selected locations
through the relay satellite 222 as shown in FIG. 22. However,
if the HAA has sufficient power available from utilization of
the ATE conversion device of the present invention, the relay
station would not be necessary. Additionally, energy har-
vested by the ATE device 100/lOOa can also be used for
internal power requirements of the HAA. The internal power
requirement is determined from the power consumption by
the PMC station movement over the guide rail, communica-
tion equipment, and system monitoring devices. Additionally,
onboard radar systems can also be operated with this energy.
The onboard radar operation can be used for monitoring any
flying objects or ground or sea level activities of interest.

The new concept HAA, as described above, has an ellipti-
cal cross-section perpendicular to the thrust axis to expand the
solar exposure area, unlike the conventional airships with a
circular cross-section. FIG. 13B shows the elliptical cross
section of an embodiment of the airship. Accordingly, the
overall shape of the new concept airship is flattened as illus-
trated in FIGS. 13A and 15. Although the elliptical cross-
section 131 of the airship 135 may be structurally less sturdy
or slightly heavier than the circular cross-section, the benefits
of the elliptical shape are greater in consideration of the lift
force and the stability of flight that might compensate the
shortcomings of elliptical cross-section. If the structural rein-
forcement of the elliptical cross section should be required to
maintain the same strength level of a circular cross sectioned,
the weight increase due to the elliptical cross section of air-
ship would be less than 20%. The near flat-top surface 135a of
the airship 135 offers a wide area to accommodate a energy
harvesting device from sun light, such as solar cells or the
advanced thermoelectric generators of the present invention.
As shown in FIG. 14, the HAA can have guide rail systems
140/146 to locate the PMC station 141 and the UAV hangers
142 to the nadir position of HAA 135. The purpose of rota-
tional capability along the guide rail 140 is to maximize the
incidence of solar flux by setting the top surface of HAA 135
always perpendicular to sun light 160. Whenever the PMC
station 141 moves on the guide rail 140, the HAA rotates the
PMC station 141 and sets the PMC station 141 at the lowest
level as shown in FIGS. 14 and 16 since the PMC station 141
is typically the heaviest unit of the HAA. Movement of PMC
141 on guide rail 140 can be accomplished through conven-
tional means (e.g. computer controlled and electric motor
driven). Similarly, in at least one embodiment, guide rails 146
can be provided, to move the UAV hangars 142 in the same
manner as the PMC station 141.

Although the invention has been described relative to spe-
cific embodiments thereof, there are numerous variations and
modifications that will be readily apparent to those skilled in
the art in light of the above teachings. It is therefore to be

US 8,529,825 B2

15

understood that, with in the scope of the appended claims, the
invention may be practiced other than as specifically
described herein.

What is claimed is:

1 . A method for forming high figure of merit thermoelectric 5
materials, comprising:

providing nanocrystals of bismuth and tellurium;
preparing a void generator material including a plurality of
nanoparticles each having a metallic outer coating,
wherein the void generator material is ferritin protein; 10
preparing a solution mixture of the bismuth nanocrystals,
tellurium nanocrystals and the void generator material;
depositing the solution mixture onto a substrate;
heating the deposited solution mixture in an oxygen envi-
ronment to create a plurality of nanovoid structures from 15
the nanoparticles resulting in a nanovoid incorporated
bismuth-tellurium film;

following the heating, treating the film to remove any oxy-
gen components remaining from heating the mixture in
the oxygen environment; and 20

causing the formation of a crystalline structure in the film.

2. The method of claim 1, wherein the metallic outer coat-
ing of the nanoparticles is selected from the group consisting
of gold, cobalt, platinum, manganese, and iron.

3. The method of claim 1, wherein the metallic outer coat- 25
ing of the nanoparticles is in the range of 3 to 4 nm thick.

4. The method of claim 1, wherein the nanoparticles are
generally spherical in shape.

5. The method of claim 1, wherein the nanoparticles have
an inner diameter of 8 nm and an outer diameter of 1 2 mn. 30

6. The method of claim 1, wherein the substrate is silicon.

7. The method of claim 1, wherein the solution mixture is

deposited on the substrate by a method selected from the
group consisting of spin-coating, dipping, and solvent cast-
ing. 35

8. The method of claim 1, wherein the oxygen environment
is 99.999% pure oxygen.

9. The method of claim 1 wherein the deposited solution
mixture is heated to no more than 400° C.

10. The method of claim 1 wherein the deposited solution 40
mixture is heated for approximately 1 hour.

11. The method of claim 1, wherein the film is treated to
remove any oxygen components remaining from heating the
mixture in the oxygen environment and fonnation of a crys-
talline structure in the film are accomplished by perfonning 45
hydrogen calcination and hydrogen plasma quenching.

16

12. A method for forming high figure of merit thermoelec-
tric materials, comprising:

providing nanocrystals of bismuth and tellurium;

preparing a void generator material including a plurality of
nanoparticles each having a metallic outer coating;

preparing a solution mixture of the bismuth nanocrystals,
tellurium nanocrystals and the void generator material;

depositing the solution mixture onto a substrate;

heating the deposited solution mixture in an oxygen envi-
ronment to create a plurality of nanovoid structures from
the nanoparticles resulting in a nanovoid incorporated
bismuth-tellurium film;

following the heating, treating the film to remove any oxy-
gen components remaining from heating the mixture in
the oxygen environment; and

causing the fonnation of a crystalline structure in the film,
wherein the film treatment and formation of a crystalline
structure in the film are accomplished by perfonning
hydrogen calcination and hydrogen plasma quenching.

13. The method of claim 12, wherein the void generator
material is an organic material.

14. The method of claim 12, wherein the void generator
material is ferritin protein.

15. The method of claim 12, wherein the metallic outer
coating of the nanoparticles is selected from the group con-
sisting of gold, cobalt, platinum, manganese, and iron.

16. The method of claim 12, wherein the metallic outer
coating of the nanoparticles is in the range of 3 to 4 nm thick.

17. The method of claim 12, wherein the nanoparticles are
generally spherical in shape.

18. The method of claim 12, wherein the nanoparticles
have an inner diameter of 8 mn and an outer diameter of 12
mn.

19. The method of claim 12, wherein the substrate is sili-
con.

20. The method of claim 12, wherein the solution mixture
is deposited on the substrate by a method selected from the
group consisting of spin-coating, dipping, and solvent cast-
ing.

21. The method of claim 12, wherein the oxygen environ-
ment is 99.999% pure oxygen.

22. The method of claim 12 wherein the deposited solution
mixture is heated to no more than 400° C.

23. The method of claim 12 wherein the deposited solution
mixture is heated for approximately 1 hour.